You can't change back to "abc123", but asks you to change it to something unique yet again -

This tells me their are useless tables of past passwords, storing it just to tell the user a big "fuck you, use another password".

p.s: I haven't logged on for at least 1 week avoiding the website, just because I did not want to change my password yet again. However, I found time by carrying my laptop while taking a big dump, and change the password in my bathroom.